1. Field of the Invention
The present invention relates to a radiation imaging device and a radiation imaging method, and in particular, to a radiation imaging device and a radiation imaging method that each detect, as an image, radiation rays that have passed through a subject, and image the same subject consecutively plural times.
2. Description of the Related Art
In the imaging of radiation images, a technique is known in which the same region of an imaging subject is imaged at different tube voltages, and the images obtained at the imaging of each tube voltage are weighted, and a differential is calculated, thereby emphasizing in the image one of image portions corresponding to hard structures such as bones or the like, or image portions corresponding to soft structures, and removing the other, thereby obtaining an energy subtraction image.
As a method of creating energy subtraction images, a conventional technique is known in which a photo-stimulable luminescent body is used with an X-ray detector, and a filter such as a copper plate or the like is placed between two detectors, such that two images with different tube voltages are substantially obtained from a single imaging, and an energy subtraction image is created from these images.
Further, an X-ray system that uses a semiconductor detector has come to be used, and it has become possible to take consecutive X-ray images in a short period of time and create energy subtraction images from two images taken consecutively. However, due to a photoconductive effect at a semiconductor detector, image information of the image taken at a first imaging remains at the detector (hereafter sometimes referred to as an “afterimage”), and this affects the image taken at the second imaging. Japanese Patent Application Laid-Open (JP-A) No. 2002-243860 discloses a technique in which, when two consecutive images are taken, the first imaging is performed at a low dose of radiation, and the second imaging is performed at a high dose of radiation, and a period of time between each imaging is reduced, taking into consideration for the photoconductive effect.
JP-A No. 2004-261489 discloses a technique in which, when two consecutive images are taken, the first imaging is performed at a low dose of radiation and image information is read in a high-speed, low-definition mode, and the second imaging is performed at a high dose of radiation, and image information is read in a low-speed, high-definition mode, thereby reducing the period of time between imagings and reducing the effect of movement of the subject's body (motion artifacts).
Further, JP-A No. 2003-284710 discloses performing a first imaging at a low dose of radiation, and obtaining a first image data aggregate, and after reducing gain and increasing image acquiring resolution, performing a second imaging at a high dose of radiation and obtaining a second image data aggregate, and obtaining a first and a second offset image.
Moreover, as a technique related to the above, JP-A No. 2002-325756 discloses observing the cardiac cycle of a patient and finding a first cardiac trigger, and imaging the patient with a first X-ray energy according to the first cardiac trigger and forming a first X-ray image, observing the cardiac cycle of a patient and finding a second cardiac trigger, and imaging the patient with a second X-ray energy according to the second cardiac trigger, and forming a second X-ray image.
The above-mentioned techniques each reduce motion artifacts in an energy subtraction image. In particular, the above-disclosed techniques aim to reduce motion artifacts by reducing an imaging time interval between plural imagings as much as possible. In this regard, images obtained by plural imagings to create an energy subtraction image are desired to use not only for the creation of an energy subtraction image, but also for diagnosis and the like. However, although a high quality image is desired for diagnosis, a decrease in an imaging time interval is linked to a decrease in image quality, for example, as in the high-speed, low-definition mode of the above-mentioned JP-A No. 2004-261489, and thus reducing an imaging time interval and increasing image quality have been conflicting goals.